It depends on the phone you have. Verizon phones don't take sim cards, so you wouldn't be able to use your T-mobile sim card. You can usually find an unlock code on the internet. There are also cables you can buy to unlock the phones. That's as much info. as I can give you without a model number.
Whats your phone brand and model ? To unlock your phone, use unlock code . you can get it from any of the online vendors like UnBBLock.com to unlock it . If your phone is other than Blackberry, get code from SimpleUnlocking.com